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| Modulated reconnection rate and energy conversion at the magnetopause under steady IMF conditions |
| We use the multi-spacecraft mission Cluster to make observational estimates of the local energy conversion across the dayside high-latitude magnetopause. The energy conversion is estimated during eleven complete magnetopause crossings under steady south-dawnward interplanetary magnetic field (IMF). We describe a new method to determine the reconnection rate from the magnitude of the local energy conversion. The reconnection rate as well as the energy conversion varies during the course of the eleven crossings and is typically much higher for the outbound crossings. This supports the previous interpretation that reconnection is continuous but its rate is modulated. |
| Publication date: 30 Apr 2008 |
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| Cluster observations of an ion-scale current sheet in the magnetotail under the presence of a guide field |
| We report on Cluster observations of a thin current sheet interval under the presence of a strong |BY| during a fast earthward flow interval between 1655 UT and 1703 UT on 17 August 2003. The strong |BY| in the tail could be associated with a strong IMF |BY|, but the large fluctuations in BY, not seen in the IMF, suggest that a varying reconnection rate causes a varying transport of BY-dominated magnetic flux and/or a change in BY due to the Hall-current system. During the encounter of the high-speed flow, an intense current layer was observed around 1655:53 UT with a peak current density of 182 nA/m², the largest current density observed by the Cluster four-spacecraft magnetic field measurement in the magnetotail. The half width of this current layer was estimated to be ~290 km, which was comparable to the ion-inertia length. Its unique signature is that the strong current is mainly field-aligned current flowing close to the center of the plasma sheet. The event was associated with parallel heating of electrons with asymmetries, which suggests that electrons moving along the field lines can contribute to a strong dawn-to-dusk current when the magnetotail current sheet becomes sufficiently thin and active in a strong guide field case. |
| Publication date: 25 Apr 2008 |
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| Study of reconnection-associated multiscale fluctuations with Cluster and Double Star |
| The objective of the paper is to asses the specific spectral scaling properties of magnetic reconnection associated fluctuations/turbulence at the earthward and tailward outflow regions observed simultaneously by the Cluster and Double Star (TC-2) spacecraft on 26 September 2005. Systematic comparisons of spectral characteristics, including variance anisotropy and scale-dependent spectral anisotropy features in wave vector space were possible due to the well-documented reconnection events, occurring between the positions of Cluster (X = - 14-16 Re) and TC-2 (X = - 6.6 Re). Another factor of key importance is that the magnetometers on the spacecraft are similar. The comparisons provide further evidence for asymmetry of physical processes in earthward/tailward reconnection outflow regions. Variance anisotropy and spectral anisotropy angles estimated from the multiscale magnetic fluctuations in the tailward outflow region show features which are characteristic for magnetohydrodynamic cascading turbulence in the presence of a local mean magnetic field. The multiscale magnetic fluctuations in the earthward outflow region are not only exhibiting more power, lack of variance, and scale-dependent anisotropies but also are having larger anisotropy angles. In this region the magnetic field is more dipolar and the main processes driving turbulence are flow breaking/mixing, perhaps combined with turbulence ageing and noncascade-related multiscale energy sources. |
| Publication date: 25 Apr 2008 |
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| Modeling PSBL high speed ion beams observed by Cluster and Double Star |
| On October 8, 2004, the Cluster and Double Star spacecraft crossed the near-Earth (12-19 RE) magnetotail neutral sheet during the recovery phase of a small, isolated substorm. Although they were separated in distance by ~7 RE and in time by ~30 min, both Cluster and Double Star observed steady, but highly structured Earthward moving >1000 km/s high speed H+ beams in the PSBL. This paper utilizes a global magnetohydrodynamic (MHD) simulation driven by Wind spacecraft solar wind input to model the large-scale structure of the PSBL and large-scale kinetic (LSK) particle tracing calculations to investigate the similarities and differences in the properties of the observed beams. This study finds that the large-scale shape of the PSBL is determined by the MHD configuration. On smaller scales, the LSK calculations, in good qualitative agreement with both Cluster and Double Star observations, demonstrated that the PSBL is highly structured in both time and space, on time intervals of less than 2 min, and spatial distances of the order of 0.2-0.5 RE. This picture of the PSBL is different from the ordered and structured region previously reported in observations. |
| Publication date: 14 Apr 2008 |
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| Comparison of periodic substorms at Jupiter and Earth |
| The Energetic Particles Detector and magnetometer measurements on Galileo showed that the Jovian magnetosphere undergoes reconfiguration processes which are very similar to the characteristics of a terrestrial substorm. At Jupiter the reconfiguration process occurs quasi-periodically with a repetition period of several days. In the terrestrial magnetosphere periodic substorms have been observed during magnetic storms. The comparison of the periodic magnetospheric disturbances at Jupiter and Earth shows that they are similar in dynamic features as well as in spatial distribution but have different energy sources. In the case of Earth, the well-established energy source is the solar wind. In the case of the Jovian magnetosphere, it is believed that internal energy is supplied by the fast planetary rotation and the moon Io which releases ~1000 kg s-1 of plasma into the magnetosphere. It is established that the energy accumulation and subsequent release lead to similar features in the magnetospheres of both planets. The particle data show periodic intensity fluctuations and plasma pressure variations. In addition, recurring signatures of stretching and dipolarization are observed in the magnetic field at the terrestrial and Jovian magnetospheres. Furthermore, the release process is associated with an intensification of auroral emissions. The typical phases for terrestrial substorms like growth, expansion and recovery are also found in the periodic substorms at Jupiter. As a lesson taken from the Jovian magnetosphere it is proposed that under certain conditions periodic magnetospheric substorms at Earth can be driven by mass-loading from the plasmasphere. |
| Publication date: 12 Apr 2008 |
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| Study of waves in the magnetotail region with cluster and DSP |
| The study of the neutral sheet is of fundamental importance in understanding the dynamics of the Earth's magnetosphere. From the earliest observation of the magnetotail, it has been found that the neutral sheet frequently appears to be in motion due to changing solar wind conditions and geomagnetic activity. Multiple crossings of the neutral sheet by spacecraft have been attributed to a flapping motion of the neutral sheet in the north-south direction, a wavy profile either along the magnetotail or the dawn-dusk direction. Cluster observations have revealed that the flapping motions of the Earth's magnetotail are of internal origin and that kink-like waves are emitted from the central part of the tail and propagate toward the tail flanks. This flapping motion is shown here to propagate at an angle of ~45° with xGSM. A possible assumption that the flapping could be created by a wake travelling away from a fast flow in the current sheet is rejected. Other waves in the magnetotail are found in the ULF range. One conjunction event between Cluster and DoubleStar TC1 is presented where all spacecraft show ULF wave activity at a period of approximately 5 min during fast Earthward flow. These waves are shown to be Kelvin-Helmholtz waves on the boundaries of the flow channel. Calculations show that the conversion of flow energy into magnetic energy through the Kelvin-Helmholtz instability can contribute to a significant part of flow breaking between Cluster and DoubleStar TC1. |
| Publication date: 11 Apr 2008 |
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| Cluster multispacecraft determination of AKR angular beaming |
| Simultaneous observations of AKR emission using the four-spacecraft Cluster array were used to make the first direct measurements of the angular beaming patterns of individual bursts. By comparing the spacecraft locations and AKR burst locations, the angular beaming pattern was found to be narrowly confined to a plane containing the magnetic field vector at the source and tangent to a circle of constant latitude. Most rays paths are confined within 15° of this tangent plane, consistent with numerical simulations of AKR k-vector orientation at maximum growth rate. The emission is also strongly directed upward in the tangent plane, which we interpret as refraction of the rays as they leave the auroral cavity. The narrow beaming pattern implies that an observer located above the polar cap can detect AKR emission only from a small fraction of the auroral oval at a given location. This has important consequences for interpreting AKR visibility at a given location. It also helps re-interpret previously published Cluster VLBI studies of AKR source locations, which are now seen to be only a subset of all possible source locations. The observations are inconsistent with either filled or hollow cone beaming models. |
| Publication date: 09 Apr 2008 |
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| Cluster observations of particle acceleration up to supra-thermal energies in the cusp region related to low-frequency wave activity - possible implications for the substorm initiation process |
| The purpose of our study is to investigate the way particles are accelerated up to supra-thermal energies in the cusp diamagnetic cavities. For this reason we have examined a number of Cluster cusp crossings, originally identified by Zhang et al. (2005), for the years 2001 and 2002 using data from RAPID, STAFF, EFW, CIS, PEACE, and FGM experiments. In the present study we focus on two particular cusp crossings on 25 March 2002 and on 10 April 2002 which demonstrate in a clear way the general characteristics of the events in our survey. Both events exhibit very sharp spatial boundaries seen both in CNO (primarily single-charged oxygen of ionospheric origin based on CIS observations) and H+ flux increases within the RAPID energy range with the magnetic field intensity being anti-correlated. Unlike the first event, the second one shows also a moderate electron flux increase. The fact that the duskward electric field Ey has relatively low values <5 mV/m while the local wave activity is very intense provides a strong indication that particle energization is caused primarily by wave-particle interactions. The wave power spectra and propagation parameters during these cusp events are examined in detail. It is concluded that the high ion fluxes and at the same time the presence or absence of any sign of energization in the electrons clearly shows that the particle acceleration depends on the wave power near the local particle gyrofrequency and on the persistence of the wave-particle interaction process before particles escape from cusp region. Furthermore, the continuous existence of energetic O+ ions suggests that energetic O+ populations are of spatial nature at least for the eight events that we have studied so far. |
| Publication date: 26 Mar 2008 |
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| Structure of the near-Earth plasma sheet during tailward flows |
| A detailed analysis of successive tailward flow bursts in the near-Earth magnetotail (X~-19 RE) plasma sheet is performed on the basis of in-situ multi-point observations by the Cluster spacecraft on 15 September 2001. The tailward flows were detected during a northward IMF interval, 2.5 h after a substorm expansion. Each flow burst (Vx<300 km/s) was associated with local auroral activation. Enhancements of the parallel and anti-parallel ~1 keV electron flux were detected during the flows. The spacecraft configuration enables to monitor the neutral sheet (Bx~0) and the level of Bx~10-15 nT simultaneously, giving a possibility to distinguish between closed plasmoid-like structures and open NFTE-like surges. The data analysis shows NFTE-like structures and localized current filaments embedded into the tailward plasma flow. 3-D shapes of the structures were reconstructed using the four-point magnetic filed measurements and the particle data. |
| Publication date: 26 Mar 2008 |
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| Two-stage oscillatory response of the magnetopause to a tangential discontinuity/vortex sheet followed by northward IMF: Cluster observations |
| We discuss the motion and structure of the magnetopause/boundary layer observed by Cluster in response to a joint tangential discontinuity/vortex sheet (TD/VS) observed by the Advanced Composition Explorer spacecraft on 7 December 2000. The observations are then supplemented by theory. Sharp polarity reversals in the east-west components of the field and flow By and Vy occurred at the discontinuity. These rotations were followed by a period of strongly northward interplanetary magnetic field (IMF). These two factors elicited a two-stage response at the magnetopause, as observed by Cluster situated in the boundary layer at the duskside terminator. First, the magnetopause suffered a large deformation from its equilibrium position, with large-amplitude oscillations of ~3-min period being set up. These are argued to be mainly the result of tangential stresses associated with DeltaVy the contribution of dynamic pressure changes being small in comparison. This strengthens recent evidence of the importance to magnetospheric dynamics of changes in azimuthal solar wind flow. The TD/VS impact caused a global response seen by ground magnetometers in a magnetic local time range spanning at least 12 h. The response monitored on ground magnetometers is similar to that brought about by magnetopause motions driven by dynamic pressure changes. Second, Cluster recorded higher-frequency waves (~79 s). Two clear phases could be distinguished from the spectral power density, which decreased by a factor of ~3 in the second phase. Applying compressible linearized MHD theory, we show that these waves are generated by the Kelvin-Helmholtz (KH) instability. Varying the local magnetic shear at the Cluster locale, as suggested by the temporal profile of the IMF clock angle, we find that locally stability was reinstated, so that the reduced power in the second phase is argued to be due residual KH activity arriving from locations farther to the dayside. |
| Publication date: 19 Mar 2008 |
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| Determination of reconnected flux via remote sensing |
| Magnetic reconnection is one of the most fundamental processes in the magnetosphere. We present here a simple method to determine the essential parameters of reconnection such as reconnected flux and location of the reconnection site out of single spacecraft data via remote sensing. On the basis of a time-dependent reconnection model, the dependence of the reconnected flux on the magnetic field z-component Bz is shown. The integral of Bz over time is proportional to the reconnected flux and depends on the distance between the reconnection site and the actual position where Bz is measured. This distance can be estimated from analysis of magnetic field Bz data. We apply our method to Cluster measurements in the Earth's magnetotail. |
| Publication date: 15 Mar 2008 |
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| Statistical properties of tail plasma sheet electrons above 40 keV |
| We use data from four tail seasons (2001-2004) of the Cluster spacecraft to study statistical features of energetic electrons (40-400 keV) in the magnetotail plasma sheet in combination with plasma parameters and indices of geomagnetic activity. We find that the horizontal magnetic field magnitude best orders the energetic electron observations. Using this method we observe a statistical flux gradient toward the neutral sheet. The fluxes also increase with an increase in the vertical magnetic field component (dipolarization). The intensity of supra-thermal electrons is observed to increase strongly as function of plasma temperature. Although the energetic electron fluxes also appear to increase strongly with geomagnetic activity this is found to be mainly due to the increase in plasma temperature with increasing Kp. Investigation of neutral sheet fluxes and phase space densities at fixed first adiabatic invariant (mu) indicates that adiabatic heating can explain observed flux increases during field dipolarization. The spectral slope in the supra-thermal range is observed to be independent of geomagnetic activity (Kp), while there is a significant local time dependence, with harder spectra observed at dawn compared to the dusk side. |
| Publication date: 11 Mar 2008 |
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| Small-Scale Energy Cascade of the Solar Wind Turbulence |
| Magnetic fluctuations in the solar wind are distributed according to Kolmogorov's power law f -5/3 below the ion cyclotron frequency fci. Above this frequency, the observed steeper power law is usually interpreted in two different ways, as a dissipative range of the solar wind turbulence, or another turbulent cascade, the nature of which is still an open question. Using the Cluster magnetic data we show that after the spectral break the intermittency increases toward higher frequencies, indicating the presence of nonlinear interactions inherent to a new inertial range and not to the dissipative range. At the same time the level of compressible fluctuations rises. We show that the energy transfer rate and intermittency are sensitive to the level of compressibility of the magnetic fluctuations within the small-scale inertial range. We conjecture that the time needed to establish this inertial range is shorter than the eddy-turnover time, and is related to dispersive effects. A simple phenomenological model, based on the compressible Hall MHD, predicts the magnetic spectrum ~k-7/3 + 2 alpha, which depends on the degree of plasma compression alpha. |
| Publication date: 20 Feb 2008 |
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| Extended SuperDARN and IMAGE observations for northward IMF: Evidence for dual lobe reconnection |
| We present observations of ionospheric convection in the Northern Hemisphere made by the SuperDARN radar network during a 3 h period on 3 December 2001. The interplanetary magnetic field (IMF) during the time of observations is predominately northward with the By component changing from positive to slightly negative. During this period Cluster is skimming the southern high latitude dusk magnetopause and reveals that reconnection is going on quasi-continuously with the reconnection site being most of the time tailward of the southern cusp and always near the satellite location (Retinò, et al., 2005). Detailed analysis of the three dimensional distribution function indicates that Cluster samples magnetosheath lines connected with geomagnetic field lines tailward of the cusps in both hemispheres (Bavassano Cattaneo et al., 2006). The evolution of the ionospheric convection measured by SuperDARN, together with IMAGE FUV observations of aurorae and DMSP particle precipitation data, confirms Cluster observations and shows that simultaneous reconnection poleward of both the northern and southern cusps occurs at a variable rate on the dusk part of the magnetosphere when the IMF clock angle is small. |
| Publication date: 06 Feb 2008 |
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| An assessment of the role of the centrifugal acceleration mechanism in high altitude polar cap oxygen ion outflow |
| The role of the centrifugal acceleration mechanism for ion outflow at high altitude above the polar cap has been investigated. Magnetometer data from the four Cluster spacecraft has been used to obtain an estimate of magnetic field gradients. This is combined with ion moment data of the convection drift and the field-aligned particle velocity. Thus all spatial terms in the expression for the centrifugal acceleration are directly obtained from observations. The temporal variation of the unit vector of the magnetic field is estimated by predicting consecutive measurement-points through the use of observed estimates of the magnetic field gradients, and subtracting this from the consecutively observed value. The calculation has been performed for observations of outflowing O+ beams in January to May for the years 2001-2003, and covers an altitude range of about 5 to 12 RE. The accumulated centrifugal acceleration during each orbit is compared with the observed parallel velocities to get an estimate of the relative role of the centrifugal acceleration. Finally the observed spatial terms (parallel and perpendicular) of the centrifugal acceleration are compared with the results obtained when the magnetic field data was taken from the Tsyganenko T89 model instead. It is found that the centrifugal acceleration mechanism is significant, and may explain a large fraction of the parallel velocities observed at high altitude above the polar cap. The magnetic field model results underestimate the centrifugal acceleration at the highest altitudes investigated and show some systematic differences as compared to the observations in the lower altitude ranges investigated. - Remainder of abstract truncated - |
| Publication date: 04 Feb 2008 |
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| EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary |
| The dynamics of the polar cap boundary and auroral oval in the nightside ionosphere are studied during late expansion and recovery of a substorm from the region between Tromsø (66.6° cgmLat) and Longyearbyen (75.2° cgmLat) on 27 February 2004 by using the coordinated EISCAT incoherent scatter radar, MIRACLE magnetometer and Cluster satellite measurements. During the late substorm expansion/early recovery phase, the polar cap boundary (PCB) made zig-zag-type motion with amplitude of 2.5° cgmLat and period of about 30 min near magnetic midnight. We suggest that the poleward motions of the PCB were produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL). The subsequent equatorward motions of the PCB would then represent the recovery of the merging line towards the equilibrium state (Cowley and Lockwood, 1992). The observed bursts of enhanced westward electrojet just equatorward of the polar cap boundary during poleward expansions were produced plausibly by particles accelerated in the vicinity of the neutral line and thus lend evidence to the Cowley-Lockwood paradigm.
- Remainder of abstract truncated - |
| Publication date: 04 Feb 2008 |
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| Observed tail current systems associated with bursty bulk flows and auroral streamers during a period of multiple substorms |
| We present a multi-instrument study of a substorm bursty bulk flow (BBF) and auroral streamer. During a substorm on 25 August 2003, which was one of a series of substorms that occurred between 00:00 and 05:00 UT, the Cluster spacecraft encountered a BBF event travelling Earthwards and duskwards with a velocity of ~500 kms-1 some nine minutes after the onset of the substorm. Coincident with this event the IMAGE spacecraft detected an auroral streamer in the substorm auroral bulge in the Southern Hemisphere near the footpoints of the Cluster spacecraft. Using FluxGate Magnetometer (FGM) data from the four Cluster spacecraft, we determine the field-aligned currents in the BBF, using the curlometer technique, to have been ~5 mA km-2. When projected into the ionosphere, these currents give ionospheric field-aligned currents of ~18 A km-2, which is comparable with previously observed ionospheric field-aligned currents associated with BBFs and auroral streamers. The observations of the BBF are consistent with the plasma "bubble" model of Chen and Wolf (1993). Furthermore, we show that the observations of the BBF are consistent with the creation of the BBF by the reconnection of open field lines Earthward of a substorm associated near-Earth neutral line.
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| Publication date: 04 Feb 2008 |
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| High-latitude Earth's magnetopause outside the cusp: Cluster observations |
| We investigated the ambient plasma and magnetic field conditions at high latitudes, as well as the macroparameters of the magnetopause. For this purpose we used Cluster spacecraft plasma and magnetic field data when all the interspacecraft distances were less than 300 km. We analyzed 154 magnetosheath-magnetosphere transitions which allow to distinguish different boundaries between the magnetosphere and the magnetosheath. First, we found transitions similar to the low-latitude boundary layer, the plasma mantle, and cusp-associated transitions. Second, we estimated the length of these transitions. Third, we found with high statistical evidence sub-Alfvénic magnetosheath plasma flows just above the plasma mantle. These flows are supposed to stabilize magnetopause reconnection. Fourth, we carried out an analysis of the magnetopause pressure balance. We found a group of 24 transitions during which both the thermal and the magnetic magnetosheath pressure exceeded the magnetospheric pressure, providing conditions for unusual magnetopause formation. Fifth, for 52 magnetopause crossings we obtained the orientation as well as distributions of velocity, thickness, and current density of the magnetopause. It was found that the magnetopause with an attached plasma mantle moves slower, is thinner, and reaches higher current densities than the one with an adjacent low-latitude-like boundary layer. A comparison with the magnetopause at low latitudes revealed that the high-latitude magnetopause is about two times thicker. |
| Publication date: 31 Jan 2008 |
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| Magnetic double gradient mechanism for flapping oscillations of a current sheet |
| A new kind of magnetohydrodynamic waves are analyzed for a current sheet in a presence of a small normal magnetic field component varying along the sheet. As a background, two simplified models of a current sheet are considered with a uniform and nonuniform current distributions in the current sheet. On a basis of these two models, the flapping-type waves are obtained which are related to a coexistence of two gradients of the tangential and normal magnetic field components along the normal and tangential directions with respect to the current sheet. A stable situation for the current sheet is associated with a positive result of the multiplication of the two magnetic gradients, and unstable (wave growth) condition corresponds to a negative result of the product. In the stable region, the "kink"-like wave mode is interpreted as so called flapping waves observed in the Earth's magnetotail current sheet. |
| Publication date: 31 Jan 2008 |
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| Observations of the development of electron temperature anisotropies in Earth's magnetosheath |
| Electron velocity distributions in Earth's magnetosheath exhibit a number of nonequilibrium characteristics, including a flat-topped shape at thermal energies and anisotropies relative to the magnetic field, with Tperp. e > Tparallel e. These features are related to processes at the bow shock and within the magnetosheath. In the present investigation we focus our attention on the development of the electron temperature anisotropy in the magnetosheath, employing the simultaneous multispacecraft data obtained by the Plasma Electron and Current Experiment (PEACE) on board Cluster. Such studies remove the temporal ambiguities introduced by single-spacecraft analyses. We find that the electron velocity space distributions just behind the bow shock are nearly isotropic with a slight Tperp. e > Tparallel e anisotropy, whereas deeper into the magnetosheath the electrons exhibit a significant Tperp. e > Tparallel e anisotropy. We find observational evidence for two processes that contribute to the sheath anisotropy. There is a clear decrease of suprathermal electrons at 0° and 180° pitch angles, suggesting that this population suffers losses (e.g., into the upstream region) related to the global configuration. Additionally, an ongoing local mechanism inflates the 90° pitch angle suprathermal electron population. |
| Publication date: 31 Jan 2008 |
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